The present invention relates to a self-adjusting thread braking device for units for feeding the weft to textile machines, in particular shuttle-less, gripper and bullet looms.
As is known, weft feeders are units which comprise a fixed cylindrical drum on which a rotating arm winds a plurality of turns of thread which constitute a feed reserve, means for causing the advancement of the reserve turns from the base to the end of the drum and means for braking in output the thread which unwinds from the drum and feeds the loom.
The evolution of shuttle-less looms, as a consequence of which significant increases in the amounts (meters) of weft thread inserted in the unit time (minute) have been achieved, entails considerable thread braking problems for the solution of which known braking means have proved to be fully inadequate.
Two kinds of braking means, both mechanical, have substantially been used so far: brush brakes and those with metallic laminas. Brush brakes of the first kind are constituted by an annular set of bristles, typically made of synthetic fibers, which is arranged inside a ring which surrounds the fixed drum. The bristles are in contact with the drum and brake the thread which unwinds from it with their elastic action. This type of brake, which is very effective in terms of balloon reduction, performs a modest braking action which is matched by modest thread tension but, most of all, said action is discontinuous and rapidly degrades due to the wear of the bristles and/or to their clogging caused by dust and lint. It is therefore generally used in combination with a disk brake or with a brake with opposite metallic laminas which however, besides also being subject to clogging, introduces evident structural complications and other problems specified hereinafter.
Brakes of the second type, which comprise a plurality of individual laminas which elastically engage the drum of the unit, are partially free from these problems and essentially perform a stronger braking action, but at the price of considerable structural complication of the braking element and of discontinuity in the braking action due to the transfer of the thread from one lamina to the next.
Furthermore, the cyclic passage of the thread beneath the individual laminas fatigue-stresses said laminas; the stress is greater as the count of the thread increases, and this causes the breakage of the laminas in a relatively short time.
But the greatest problem, which is common to both of the above known brake types, is constituted by the fact that the braking action exerted on the thread depends on the advancement speed of said thread and increases in an approximately linear manner together with said speed due to the fact that, in these systems, the friction coefficient .mu. between the braking means and the thread varies correspondingly according to the speed. Typically, the diagram of the speeds of the grippers of a modern loom is approximately sinusoidal with two half-periods per beat cycle. Consequently, speed passes from a null value during the swapping of the weft between the clamps to a maximum value during weft traction.
For correct weaving, the thread must be subjected to tension during the entire beating cycle. In particular, the thread must be subjected to an adequate tension, hereinafter termed static tension, even when the speed of the grippers becomes zero. Static tension is set by acting on the elements for adjusting the braking means; said elements vary the contact pressure between the braking means and the thread. Said pressure cannot drop below a certain value, to prevent failure to transfer the weft between the grippers and/or the presence of loose wefts on the side of the piece of fabric at which the pulling gripper releases said weft. As the speed increases, said static tension, set to the minimum value which is compatible with these requirements, reaches values which are much higher in percentage, and in modern looms increases of 700% in static tension can easily be reached, with the consequent easy and frequent breakage of the weft thread.
In order to try to obviate this severe problem, it has been proposed to modulate the braking action of the braking means by varying the contact pressure of said braking means by virtue of an electromagnetic device which is supplied with an electric current which varies according to the speed of the loom. The prior international patent application published as no. WO 91/14032 illustrates a device of this type whose use, however, is very onerous, since it requires a current supply means which can consistently follow the speed variations of the loom. Furthermore, said known electromagnetic device on one hand significantly complicates the structure of the weft feed unit and on the other hand, due to the inertia of the braking system, is not fully satisfactory in terms of the adjustment of the braking action which said system applies to the thread.